KEY CONCEPT Genes encode proteins that produce a diverse range of traits.

The same gene can have many versions. A gene is a piece of DNA that directs a cell to make a certain protein. The locus is a specific position of the gene on a pair of homologous chromosomes.

An allele is any alternative form of a gene occurring at a specific locus on a chromosome. You have two alleles for each gene. Each parent donates one allele for every gene. Homozygous describes two alleles that are the same at a specific locus. Heterozygous describes two alleles that are different at a specific locus.

Genes influence the development of traits. All of an organism’s genetic material is called the genome. A genotype refers to the makeup of a specific set of genes. A phenotype is the physical expression of a trait.

Alleles can be represented using letters. A dominant allele is expressed as a phenotype when at least one allele is dominant. (Uppercase letters) A recessive allele is expressed as a phenotype only when two copies are present. (Lowercase letters)

Homozygous dominant : TT Heterozygous: Tt Both homozygous dominant and heterozygous genotypes yield a dominant phenotype. Homozygous recessive: tt Need both copies of recessive gene to yield the recessive phenotype. Most traits occur in a range and do not follow simple dominant-recessive patterns.